Electrical compression system



Feb. 23, 1943.

B. F. MILLER 2312,260

ELECTRICAL COMPRESSION SYSTEM Fled May 28, 1941 2 Sheets-Sheet 1 Feb.23, 1.943. B. MILLER 2 ELECTRICAL COMPRESSION SYSTEM Filed May 28, 19412 Sheets-Sheet 2 Patented Feb. 23, 1943 ELEOTRICAL COMPRESSION SYSTEMBurton F. Millei, Los Angeles, Calli., asslgnor to Radio Corporatlon ofAmerica, a corporatlon of Delaware Application May 28, 1941, Sel'il No.395,563

13 Claims.

This lnvntion relatesto electrical current transmission systems andparticularly to transmission systems wherein the amplitudes of thecurrents are decreased or increased in amounts dependent upon theiroriginal amplitudes, such amplitude variations being commonly known ascompresson and expansion, respectively.

In the art of signal current transmission, particularly in the audiorange of 'crequencies, it

has beeniound that by reducing the higher amplitudes of certain signalsto a greater degree than their lower amplitudes, beneficial results areobtained. This is particularly true where the audio signals arereproduced in auditoriums or theatres by public address or sound motionpicture film systems.

It is well known that an original sound signal source may have a levelor amplitude range of the order of 40 to 80 db., but it has frequentlybeen found undesirable to reproduce such 21 volume range in theatres andhomes. For instance, in the reproduction of sound in theatres, theminimum level of reprduction is determined by the level of the theatrebackground noise produced by the audience or by theatre apparatus whlchmay be operating, such as air-conditioning systems, etc. Therefore, theminimum sound reproduction level must be set at a higher value than thetheatre noise level or it would otherwise be masked and. renderedunintelligible. Thus, a certain minimum level is fixed, and thereproducin-g system must be set to reproduce this minimum sound levelfor the lowest level passages recorded. this minimum sound level usuallybeing higher than the lowest level of the original sound.

'Iherefore, since the lowest level of reproduction is generally righerthan the lowest level of the original signa1, it is obvious that with alinear relationship existing between reproduced levels and the originallevels the highest level of reproduction will exceed the highest levelof the original signal by a corresponding ratio. It has been found thatsuch amaximum signal level is entirely too loud for theatre orauditorium reproduction purposes since it is not only annoying to thelisteners, but may even be painful. It is t hus desirable to introduceinto the original signal channel means of compressing the volume rangeof the signal being transmitted.

In sound pictures this compression may be introduced either during therecording or the reproduction of the film, while in public addresssystems, it may be introduced at some point between the microphoneand-the loudspeaker. is also applicable to radio broadcast systems.

Thus, the use of compression a 3complished by varying the relativeamplitudes of the signal in accordance with their original levels hasbeen adopted, particularly in sound recording for 1110-- tien pictures,the variation in level being accomplishea by the use of variable gainampli- Signal flets in the signal transmisslon circuit. level control isusually obtained by rectifying a portion of the original signal and.applying the products of rectification to control the gein of one Of thesignal amplifiers. Such a compressor system is disclosed and claimed inU. S. Patent No. 2225683 of September 9 1941. The operation of such acompressor system is usually made dependent upon the short-periodaverage value of the signals being transmitted. Thus, the variaton inthe gain of the amplifier is proportional to the short-period energycontent of the signaling currents passing therethrough.

The term short-period as herein used, refers to the approximate timeinterval required for the transmisslon of any essentially constantamplitude porti0n of the signal wave train, and may be of the order ofone to ten milliseconds In general, since the spectral energydistribution of complex sounds is non-uniform, the degree of compressionobtained at any instant of time is actually dependent upon the amplitudeof the predominant frequency or frequencies being transmitted. Forexample, certain words are formed of a predominant vowel portion and aless prominent sibilant portion, as in the case of the word same, wherethe s portion of the word is an unvoiced component, while the remainderof the word ame is a voiced component. That is, the s portion is iormedby alr passing through the teeth and lips, while the ame portion isproduced by the vocal cords. It has been found that in the compressionof electrical currents corresponding to such a word, the small amount ofenergy of the unvoiced component is insuflicient to decrease the gein ofthe controlled amplifler to the same degree as the ame or voiced p0rtionof the word. Thus, the

resultis that the s becomes predominant upon reproduct ion in comparisonwith the remalnng portion of the word. The normal type of compressionsystem, therefore, produces a signal whose spectral energy distributionis no longer similar to that of the original sound, and which exhibits aspectral energy distribution characteristic which is a function of thesignal level being transmitted. This is 'true regardless of the order ofthe voiced and unvoiced components within the word, and constitutes abasic form of signal distortion.

In the past it has frequently been necessary in sound recording practiceto manually paint over the sound track portions of the sound recordrepresenting the unvoiced components of the signal to reduce theirexcessive amplitudes with respect to the amplitudes of the voicedcomreproduction of the original signal.. 'I'his was a very tedious.time-consuming, and costly operation. The present invention solves thisproblem by providing essentially uniform compression of all portions ofwords or phrasesregardless of their energy content or spectral energydistribution.

It is well known that the general spectra] energy distribution curvestor speech and musical sounds indicate that the bulk of the energy ofsuch sounds extends throughout the lower portion of the audio spectrum,while such sounds as the unvoiced components of speech, lying in theupper frequency portion of the audio spectrum, contain relatively smallamounts of energy. Therefore, a compressor was devised wherein thevariable gain amplifler was selectively controlled with respect to thespectra] energy distribution of the signals being compressed.

The principai object of the invention, therefore, is to improve theaction of compressors of electrical signals.

Another object of the invention is to so compress audio signals as tosubstantially retain their original spectra] energy distribution.

A further object of the invention is to provide a signal transmissionsystem wherein the signals are increased or decreased in amplitude at acertain uniform variation with respect to their original amplitudes.

A further object the invention is to vary the gain of an amplifler insuch a manner as to retain substantially the same spectra] energydistribution in the compressed signal as existed in the original signal.

Although the novel eatures which are believed to be characteristic ofthis invention are pointed out with particularity in the appendedclaims. the manner of its organization and the mode of its operationwil] be better understood by refening to the following description reedin conjunctio n with the accompanying drawings forming a part thereof,in which Fig. 1 is a diagrammatic arrangement of an eiectrical currenttransmission system embodying the invention;

Fig. 2 is a. detailed schematic drawing 01 the variable gain amplifierand contro] circuits i'or the system of Fig. 1; and

Fig. 3 is a graph illustrating the action of the system of Figs. 1 and2.

Referring now to Fig. 1, a pickup 4, which may be either a microphone,111m or disc phonograph or other source of signal current, is cormectedto an amplfier 5 which reeds a variable gain amplifler 6. The output ofthe variable gain amplifier 6 is impressed upon a power ampliiler 1 andthen on either a recorder, loudspeaker, or a transmission system Forinstance, 11' the system is a sound recording system, the pickup 4 willbe a microphone, while the output of the power amplifler wil] beimpressed on a sound recorder which may be monitored by a loudspeaker.If the invention is -emboclied in a public address system, the pickupunit 4 may he 8" microphone and the unit 2, a loudspeaker, while theinvention may be embodied in a phonograph reproducing system wherein thepickup 4 may be either a iilm soundhead or a disc phonograph pickupunit. Also corinected to the output of the variable -gain ampliiier is aselective control device II ieeding a rectiiler II, the output of therectiiier being connected to the variable gain ampliiler 6 through atiming contro1 unit i2. Although the rectifler is shown connected to theoutput of the variablegain ampliiier, it is to be understood that it mayalso be ied tromthe input thereto.

Reterring now to Fig. 2 in which the circuit details of the variablegain ampliiler 0. selective control unit IO, reotiiler Il, and timingcontrol unit l2 are illustrated. it will be observed that the genera]arrangement of the system is similar to that shown in theaboie-mentioned U. 8. Patent No. 2225683 of September 9, 1941. The inputto the variable gein ampliiier is at terminals l5 which are connected tothe primary of a push-pull transi'ormer IS, the primaries andsecondaries of which are shunted by resistances I], 18 and l8 to providesuitable terminations. The secondary of transiormer IG is connected tothe control grids oi variable mu tubes 2] and 22. The plates o! tubes 2]and 22 are coupled through a resistance capacity netwerk comprisingresistances 24, 25, 28. 2"], 28 and 29, and condensers 20 and 22, to thegrids of triode ampliiier tubes 2] and 22. The plates of ampli-- iiertubes 2! and 22 are connected to the primary of a transiormer 24, thesecondary of which is shown connected to output terminals 25.

Normal grid bias tor the variable mu tubes 2! and 22 is obtained tromthe voltage drop across a resistance 31 through which flow the plate andsereen grid currents of tubes 2! and 22 and the bleeder current of avoltage divider composed of resistances I], 28 and 29, resistances 3],

32 and 29 providing the correct potentie] to the screen grids of tubes2! and 22. Resistance 4' and condenser 48 form a decoupling filter or 35netwerk to prevent feedback due to a plate 4o tube circuits. Condenser41 is a plate power voltage supply common to tubes 2], 22, M and 22.Condenser 45 and resistance 39 form a decoupling filter or networkbetweenthe scren grid circuit of tubes 2I and 22 and the other supplyby-pass condenser. Grid bias ior tubes 3] and 22 is obtained trom thevoltage drop across a resistance 48 due to plate current flow in thesetubes.

The portion of the circuit :Iust described is the variabie gainampliiier unit shown at 6 in Fig. 1, the gain of which is varied in amanner now to be described. The voltage generated across the secondaryof transformer 34 is impressed on a selective control unit including atransformer 50, the secondary of which is shnted by a potentiometerresistance 5| having a slider 52 tor fixing the initial voltageimpressed upon the grid oi-a triode amplifler 54. Also connected in thesecondary circuit of transformer 50 is a selecting or energydiscriminating netpass condenser ior resistance 65, while a resistance61 and a condenser 6! form a decoupling filter or networlr ior the platecircuit of tube 54.

For obtaining an initial positive bias on the cathodes of rect iiier 84with respect to its plates, a voltage divider comprising resistancs 10,'I] and 12 is employed, resistance 1I being a potentiometr with anadjustable slider 'Il ior adiusting the level at whichconipression isinitiated.

The rectiiied current i'r om the rectiiier 84 flows over a conductor IIthrough a resistance 'II in series with a resistance 4I shunted by acondenser 42. The voltage drop across resistance ll controls thenegative biasing voltage applied to the control grids of varlable mutubes 2| and 22. In the compressor disclosed in the abovementioned U. S.patent, this bias voltage vari ed in accordance with the average voltagechanges across the secondary of transformer 34 and thus producedvariations in gain or transmission in accordance with the short-periodaverage value of the signal levels, as explained above. Such gainvariations of tubes 2! and 22 were thus based on the short-periodaverage energy content of the signals since they were the direct resultof a linear rectification of thesignal currents by the rectifler 64.However, as mentioned above, since the spectral energy distribution ofwords and other sounds varies in accordance with the character of thevoiced and unvoiced components thereoi', it is necessary, in order tomake the degree of control of the. amplifler independent of the spectrallocation of these components, to make the products of rectificationprovide a control voltage whose magnitude varies with the frequency ofthe predominant components of the signal being transmitted through thevariable gain amplifier in a. manner corresponding to the inversecharacteristic of the average spectral energy dstribution curve forwords and other sounds.

To explain how this is ac-complished, reference is made to Fig. 3 inwhich there are illustrated spectral energy distribution characteristicsof the voiced and unvoiced components of typical signals. The threesolid-line curves a, b and represent three typical signals and show thatthe voiced components thereof lie in the lower frequency range taken, orpurposes of illustration, as trom zero to approximately 1000 cycles,while the unvoiced components 11e in a range between 1000 cycles and10,000 or above. In general, therefore, the frequencies of the sibilantslie at the high end of the audio spectrum, while the vowel componentslie at the 1ow end.

Now, under the action of prior compressors. the compressed wave form ofany one of signals,

a, b or c will be as illustrated by the dotted curve d. This is causedby the fact that there is insufficient energy in the unvoiced componentrange to reduce the gain of the variabie gain amplifier to the sameextent as the gain is reduced by the voiced component range. There isthus introduced a very marked degree of distortion in the recording andreproduction of the signals, the effect being very noticable by thestrong emphasis placed upon the ss and other form of sibilants. Thisgives a whistling character to the sibilants when reproduced.

To enablethe compression to be properly proportionate over the entirefrequency range, the circuit including elements 60 and 6! is provided,

these elements serving to accentuate the amplias shown by the curve e,this curve being approximately the complement of the average signalcurves a, b and 0 in the unvoiced component range. Thus, instead ofcompression occurring in accordance with curve d, the signals wiil becompressed in the manner shown by the combinatlon curve d --l. It is tobe understood that under certain conditions of operation, it may bedesirable that curve e complement the average of curves a, b and 0throughoutthe entire frequency spectrum.

To illustrate a typical characteristic curve e, the following dataindicates the marmer in which the relative grid to cathode slgnalvoltage applied to tube 54 varies with irequency when constant voltageis appliedto transiormer 50:

Reiative Frequcncy voltgze in db.

As mentioned above, the variation in the volt age drop across resistance4l due to the rectified current flowing therein produces the variationin bias on tubes 2! and 22. The resistatrce H is shunted by a condenser42, which condensor has a double function. It serves as a by-passcondenser for the alternating components of the rectified current whichvaries the bias and also determines, in conjunction with resistances 4Iand 16, the internal resistance of the iectifier, and the impedance ofthe secondary of the rectifier output transformer, the rapidity withwhich voltage changes across the resistance 4! can take place. That is,a certain time constant for the controi circuit is determined by thiscombination.

Although it is known that a compressed record may be expanded duringreproduction to obtain the original sound volume range, the presentapplication of compression serves to reduce the sound volume rangethrough linear reproduction of the compressed record.

Therefore, the present invention may not be necessary when the signal isto be expanded dur ing reproduction since the reverse spectral energydistribution distortion is obtainable in the expanding reproducer.However, by the use of the invention in compression, the recorded recordmay be reproduced either in a linear manner with a standard reproduceror with a simple expanding reproducerembodying a complementary selectivecontrol netwerk. Thus, the present invention permits the record to bereproduced in either a linear or expanding type of system, whereaswithout the invention the record could be faithfully reproduced onlythrough a complementary expander. Another lfeature of the presentinvention is that it permits a compressed record to be reproduced in astandard linear reproducing system withoutspectral energy distributiondistortion, there being no type of equalization as such which wiliremovethis distortion. Thus such a distorted compressed record must bereproduced by a complementary expander system in order to faithfullyconvert the signals to their original amplitude relationship.

Aithough the invention has been shown asso ciated;with a variable gainampiifler, it is to be understood that it may also beembodied in anytype of variable transmission system using other types of variabletransmission elements.

What I claim as my invention is: 1. In en lectricel current transmissionsystem in which the amplitudes of the current are automaticallyvarled.in accordance with their original relative va.lues thecombination o! means for generating en electrical current correspondingto a. signal, a variable transmission element for sa.id current, meansfor varying the relative transmission of sa.id currents through saidtransmission element. said varying meaus including a rectifler 3. Themethod of proportionately varying the amplitude of electrical currentscorresponding to 9. signa.l having voiced and unvoiced components whichnormally r'eceive different degrees of amplification, comprising passingthe signal through 9. signal channel including en amplifier, deriving a.control signal from the signal channel having an a.verage spectralenergy distribution characteristic varying in accordance with theinverse average spectral energy distribution characteristic of thesignal, and utilizing said controi signal to vary the gein of saidamplifier to provide en output signal whose spectral energy distributionchara.cteristic corresponds to the energy distribution characteristic ofthe input signal.

4. The method of variably transmitting signals in accordance with theiramplitudes, said signals having an energy distribution dependent uponthe frequency of the components of sa.id signals, comprising passing thesignals through a signa.l channel including an amplifier, derivingcontrol signals from sa.id signal channel direotly proportional to theshort-period avera.ge energy content of said signals, said controlsignals having respective spectral energy distribution characteristicsvarying in accordance with the inverse average spectra! energydistribution characteristics of the. signals being transmitted, andutilizing sa,id control signals to vary the gain ofsaid amplifler toobtain correSpondence between the, respective spectral nergydistribution characteristics of the input and output signals.

5. A system for transmitting signaling eurrents at cmpressed amplitudescomprising a. transmission line for said currents, a variab1e gainamplifler in said line, means ior rectifying a. portion of saidsignaling currents, means for connecting the input and output of saidrectifying means to said variabie gain amplifier, and means connected inthe input to said rectifying means for obtaining a, rectifled currentfroni said signaling currents in accordance with the inverse averagespectral energy distribution of said signalling currents to provide anoutput signal whose spectral energy distribution substantiallycorresponds to the spectral energy distribution of the input signal.

6. A system in accordance with claim 5 in which said last-mentionedmenas comprises a. frequency discriminating filter.

7. A system in acc0rdanoe with claim 5 in which ineens are provided inthe output of zuid rectiiylng means or 'controlling the ra.te ofimpressin of sidrectiiied currents on said variable gein amplifler.

8. An electrioal current compressor system 101. audio signals comprisinga. va.riable gein amplifler, a rectiiier, means tor connecting theinput. and output circuits of said rectifler to said variablegainamplifier, means connected in the input circuit; ot said rectifier foxvarying the relative amplitude of impression of sa.id audio signais onsaid rectifler, said last-mentioned mea.ns varying the ratio ofrectified control signal magnitude to average rectifier signal inputmagnitude in nocordance with the inverse a.verage spectral energydistribution chara.cteristlc of sa.id signals to provide output signalswhose spectral energy distribution characteristics substantiallycorrespond to the spectral energy distribution characteristics ofrespective input signals.

9. An electrical current compressor system in accordance with claim 8 inwhich e. timing circuit is provided in the output cirouit-otsaidrectifier.

10. An electrical current compressor system in accordance with claim 8in which said last-mentioned means includes a corrective network havinga transmission characteristic approxixnating the inverse a.vragespectral energy distribution characteristic of said signals.

11. An electricl current compressor system ifor audio signals oomprisinga source of electrical signals, a variable gein amplifler or variablyamplifying said signals, a. rectifier for said signals, said rectifierproducing a directcurrent whose amplitude is proportional to theshort-period average energy distribution of said signals, means torconnecting the output -of said rectifier to said variable gein ampliflerto vary the-gein thereof in inverse proprtion to the amplitude of saiddirect current, a.ncl means in the input of said rectifler for varyingthe impression of said signals on said rectifler in a relationshipwhereby the output of said. rectiiier varies with the frequency of thepredominant components of said signals being transmitted through saidvariable gain amplifler in accordance with the inverse characteristic ofthe average spectral energy distribution curve of said signals toprovide an output signa-l whose spectral energy distributionsubstantially corresponds to the spectral energy distribution of thinput signal. 12. In an electrical signal variable gain ampliflersystern in which the spectral energy distribution of the output signaldoes not correspond with the spectral energy distribution of the inputsignal, the method of pmduoing a correspondence between the input andoutput spectra,l energy disspectral energy distn'bution of the inputsignal.

13. The method in accordance with claim 12 in which the control signalvaries the control of said variable gein amplifier ata, predeterininedrate.

